This invention relates to a method for growth of II-VI compound semiconductors, in particular, suitable for application to growth of p-type II-VI compound semiconductors.
Recently, demand has become greater and greater for semiconductor lasers capable of emitting light of short wavelengths, aiming improvements in recording density of optical disks and in resolution of laser printers, and efforts are devoted to developments of such lasers.
II-VI compound semiconductors are hopeful materials for fabricating semiconductor lasers for emitting light of short wavelengths. In particular, ZnMgSSe compound semiconductors, which are quaternary II-VI compound semiconductors, are known to be suitable materials for cladding layers and waveguide layers which are necessary for making on a GaAs substrate a semiconductor laser for emission of blue to green light in the band of wavelengths of 400 to 550 nm (for example, Electron. Lett. 28, 1798(1992)).
Heretofore, growth of a II-VI compound semiconductor wholly relied on molecular beam epitaxy (MBE) which is, however, unsatisfactory from the viewpoint of productivity. In more recent years, efforts were made to employ metallorganic chemical vapor deposition (MOCVD), which has an excellent productivity and has widely been used for growth of III-V compound semiconductors, also for growth of II-VI compound semiconductors.
On the other hand, nitrogen (N) has conventionally been most used as an acceptor impurity for making p-type ZnSe or other p-type II-VI compound semiconductors. Typically used as p-type dopants for growth of such p-type II-VI compound semiconductors are compounds such as ammonia (NH.sub.3), tertiary butylamine (t--BNH.sub.2), hydrazine (N.sub.2 H.sub.4), and so forth.
However, the N doping efficiency is very low when using NH.sub.3, t--BNH.sub.2, N.sub.2 H.sub.4 or other like compounds as p-type dopants, which means that a large amount of such dopants must be supplied for doping a sufficient amount of N. For example, it was reported that, when using NH.sub.3 as a p-type dopant, an amount of NH.sub.3 as large as thousands of .mu.mol/minutes must be supplied to see apparent doping of N (for example, J. Crystal Growth 101, 305(1990) and J. Crystal Growth 99, 413(1990)). For these reasons, it has been difficult to realize growth of p-type II-VI compound semiconductors with a sufficiently high doping concentration of N, that is, with a sufficiently high acceptor concentration. Specifically, in case of p-type ZnSe, maximum acceptor concentration has been 1.times.10.sup.16 cm.sup.-3 even with all possible efforts.
One of some reasons of N doping efficiency being low is that N is not readily incorporated in crystals under growth. It can be assumed that, due to a very high vapor pressure of nitrogen molecules (N.sub.2) at growth temperatures, most of N atoms to be doped may desorb in the form of N.sub.2 while migrating along the surface of a crystal under growth. The next statement shows that the vapor pressure of N.sub.2 at growth temperatures is actually very high. That is, the vapor pressure of N.sub.2 can be expressed by the following equation, called Antoine's Equation, EQU log(p/mmHg)=6.49594-255.821/(266.56+(T/.degree. C.))
(for example, Chemical Handbook, Basic Volume II (Chemical Society of Japan)). It is known from this equation that the vapor pressure of N.sub.2 at a typical temperature, 500.degree. C., for growth of II-VI compound semiconductors is 1.45.times.10.sup.6 Torr, which is as high as 1860 times the normal pressure (1 atm).
Methods for solving the problem of desorption of N include plasma doping which is often used for growth of II-VI compound semiconductors by MBE. This method dopes N.sub.2 in the form of plasma, which might promote its combination with other atoms on the surface of the crystal (mainly, atoms of group II elements) and hence prevent desorption of N. However, the use of plasma doping in growth of II-VI compound semiconductors by MOCVD involves a difficulty due to the excessively high pressure. That is, while the pressure within the reaction tube during growth by MOCVD ranges from the normal pressure to several Torr, even under reduced pressure, such a high pressure significantly reduces the life of plasma, and nitrogen no longer maintains the phase of plasma when reaching the surface of the substrate.
As explained above, it has been difficult to realize growth of p-type II-VI compound semiconductors with a sufficiently high acceptor concentration. This is a large obstacle to fabrication of light emitting devices such as semiconductor lasers or light emitting diodes by using II-VI compound semiconductors, and improvements have been longed for.